There are many known electrical drive units for wheelchairs, both arranged as the main power supply of the wheelchair and as auxiliary power assist units for conventional wheelchairs with push rims. Typically, a wheelchair designed primarily for full-time electrical drive tends to be heavier and therefore more cumbersome to use than the lighter conventional wheelchairs equipped with auxiliary power-assist drive units. The latter type typically include electric motors mounted either in the hub of the two main wheels of the wheelchair or as electric motor assemblies with auxiliary drive wheels mounted between the main wheels—either permanently fixed or removably fixed to the wheelchair.
An example of a relatively light-weight auxiliary drive unit is described in the European patent application EP 2729108 (A2), Motion-Based Power Assist System for Wheelchairs. It includes a drive motor unit and a single auxiliary drive wheel mounted between the main-wheels of a wheelchair. The unit can be easily connected and disconnected to a conventional wheelchair and has a motion based sensor system which adapts the drive power to a degree decided by the driver of the wheelchair. One drawback with a single auxiliary drive wheel is that the available traction may be limited when compared to drive units that drive the main wheels of the wheel chair. This is particularly noticeable in poor road conditions with slippery road surfaces.
Hub-mounted power assist motors are compact and offer good traction via the main wheels. One drawback with hub-mounted auxiliary drive motors, however, is that the weight of the motors cannot be removed if the driver wishes to use the wheelchair in an entirely conventional way by using hand power only. An example of a known hub mounted drive unit is described in U.S. Pat. No. 7,383,904 B, Auxiliary Power Unit Starting Apparatus for a Wheelchair. Other examples of hub-mounted power assist motors may be studied in European Patents EP 0 925 771 B1, Wheelchair with Auxiliary Power and EP 0945 113 B1, Auxiliary Propelling Device for Wheelchair Propelled by a Patient, respectively.
Examples of wheelchairs designed primarily for full-time electrical drive include a design described in British Patent Publication GB 1287122(A), A Foldable Invalid Chair. This prior art design typically represents many similar designs where the drive motors are positioned in parallel but not coaxially with the rotational axis of the main wheels. Other designs include drive motors positioned perpendicularly to the rotational axis of the main wheels. Cumbersome and often heavy angled transmissions are used in order to transfer necessary power to the drive wheels. As mentioned initially, these designs offer good traction but tend to add considerable weight to the wheelchair due to their bulky motors and transmissions which make them less suitable for example in situations where the wheelchair needs to be lifted.
In U.S. Pat. No. 5,234,066, Power Assisted Wheelchair, a drive unit is disclosed that offers an auxiliary drive unit that is configured to allow removal of the drive unit and folding of the wheelchair when not in use. The drive unit includes a relatively large box-shaped housing for two drive motors positioned in parallel but not coaxially with the rotational axis of the main wheels, hence needing space-consuming and heavy gear transmissions to drive both main wheels. Due to the relatively large size of the box-shaped housing and the added weight of the gear transmissions, this drive unit becomes cumbersome and heavy to handle for a user when it is to be removed from or installed into the wheelchair. Furthermore, the box-shaped drive unit is not width-adaptable to allow installation in wheelchairs of various track distances between the two main wheels, which is a desirable feature if the drive unit is to fit different wheelchairs from a plurality of wheelchair manufacturers. Lastly, the drive unit described in U.S. Pat. No. 5,234,066 does allow adjustment of the camber angle between the main drive wheels.